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<H1>ARPL -- Adjust RPL Field of Selector</H1>

<PRE>
Opcode    Instruction          Clocks    Description

63 /r     ARPL r/m16,r16       pm=20/21  Adjust RPL of r/m16 to not
                                         less than RPL of r16
</PRE>

<H2>Operation</H2>

<PRE>
IF RPL bits(0,1) of DEST < RPL bits(0,1) of SRC
THEN
   ZF := 1;
   RPL bits(0,1) of DEST := RPL bits(0,1) of SRC;
ELSE
   ZF := 0;
FI;
</PRE>

<H2>Description</H2>

The ARPL instruction has two operands. The first operand is a 16-bit
memory variable or word register that contains the value of a selector. The
second operand is a word register. If the RPL field ("requested privilege
level"--bottom two bits) of the first operand is less than the RPL field of
the second operand, the zero flag is set to 1 and the RPL field of the
first operand is increased to match the second operand. Otherwise, the zero
flag is set to 0 and no change is made to the first operand.
<P>
ARPL appears in operating system software, not in application programs. It
is used to guarantee that a selector parameter to a subroutine does not
request more privilege than the caller is allowed. The second operand of
ARPL is normally a register that contains the CS selector value of the
caller.

<H2>Flags Affected</H2>

ZF as described above

<H2>Protected Mode Exceptions</H2>

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

<H2>Real Address Mode Exceptions</H2>

Interrupt 6; ARPL is not recognized in Real Address Mode

<H2>Virtual 8086 Mode Exceptions</H2>

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


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Chapter 17 -- 80386 Instruction Set</A><BR>
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